It is well known that solar cells or photovoltaic cells can be used to convert solar energy into electric current. Typical photovoltaic cells include a substrate layer for mounting the cell and two ohmic contacts or electrode layers for passing current to an external electrical circuit. The cell also includes an active semiconductor junction, usually comprised of two or three semiconductor layers in series. The two layer type of semiconductor cell consists of an n-type layer and a p-type layer, and the three layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The photovoltaic cells operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers applies a net voltage across the two electrode layers in the photovoltaic cell, establishing a current of electricity.
Solar cells or photovoltaic cells are examples of diode structures where the light passes through a transparent electrode layer and energizes an active semiconductor junction. The diode structure can also take on a different mode, where the current is applied to the transparent electrode layers of a layered semiconductor cell, and the output is light energy. In such a case the diode structure is a light emitting diode (LED). In a specific example of this type of diode structure, the light emitting diode is incorporated into a flat panel display. Another area where the diode structure of the invention can be used is in energy efficient coatings for glass.
The semiconductor layers of diode structures may be formed from single crystalline materials, amorphous materials, or polycrystalline materials. Single crystalline layers are often made with a molecular beam epitaxy (NBE) process (or other vapor deposition process), but the largest area of a substrate that can be practically covered using such processes is on the order of several tens of square centimeters because it is limited by the size of single crystal substrates, which is an impractical size when considering the surface area required for economically practical solar cells. Therefore, although single crystal photovoltaic materials can be used to generate conversion efficiencies over 20 percent, they have significant drawbacks because of their high manufactured cost. Accordingly, where the solar cell must compete with conventional electricity generation by nuclear or fossil fuel, polycrystalline materials are viewed as the material of choice for the production of semiconductors and solar cells using such semiconductors. Typically, the polycrystalline material of choice for a semiconductor in a photovoltaic cell is a group II-group VI compound, such as cadmium telluride. Cadmium telluride is preferred for thin film photovoltaic applications because of its direct band gap of 1.5 electron volts which is well matched to the solar spectrum, and its ability to be doped n-type and p-type, which permits formation of a variety of junction structures. P-type cadmium telluride is also compatible with n-type semiconductor partners, such as cadmium sulfide, to form heterojunction solar cells.
It is known that an RF sputtering technique can be used to deposit thin films of cadmium telluride onto substrates for use in photovoltaic cells, as disclosed in U.S. Pat. No. 5,393,675 to Compaan. The RF sputtering technique can also be used for depositing other thin group II-group VI semiconductor films such as cadmium sulfide and zinc telluride for use in a photovoltaic cell. RF sputtering involves positioning a substrate in a pressure chamber and operating a planar magnetron sputtering gun. The gun includes a target (the cathode) of pressed and sintered cadmium sulfide or cadmium telluride typically prepared from powder. The substrate is positioned behind the target and is coated as the target is bombarded. The process takes place typically in an inert atmosphere of argon gas.
In most photovoltaic cells it is necessary to dope one or more semiconductor layers to be highly conductive to achieve easy flow of electrons and holes into the respective contact electrodes. Particularly for cadmium telluride and zinc telluride and related semiconductors, copper is often used for this dopant. While the doping with copper is successful in obtaining the desired conductivity, the use of copper has its limitations. It has been found that over time the copper diffuses into other semiconductor layers of the photovoltaic cell, thereby causing a loss in efficiency. When copper is used to dope a zinc telluride contacting layer the copper tends to move into the cadmium telluride layer and even penetrate into the cadmium sulfide/cadmium telluride junction where it degrades the photovoltaic activity. Further, when zinc telluride and other semiconductors are heavily doped with copper, the semiconductor layer begins to lose its transparency to radiation transmission.
One of the requirements for the transparent electrode layers that pass current to the external electrical circuit in diode structures is that the electrode layers or conductors must 1.) conduct electricity to and from the diode structure, and 2.) be substantially transparent to certain light wavelengths (typically over 400 nm) so that the solar energy can reach the primary semiconductor layers forming the active semiconductor junction. In many cases the restriction on the amount of light allowed to be passed through the conductor layer sets a practical limit on the efficiency of the photovoltaic cell. Also, the electrical conductivity of the electrode layer is an important factor in the overall efficiency of a photovoltaic cell. Diminished conductivity of the transparent electrode layers reduces the efficiency of the solar cell. It would be advantageous if there could be developed improved diode structures, such as solar cells or photovoltaic cells, and such as LED's, particularly where the improved diode structures exhibit increased efficiency due to improvements in either the transparency of the transparent electrode layers, or the conductivity of the transparent electrode layers, or both.